1. Field of the Invention
The present invention relates to an elevator control method and to an apparatus for controlling an elevator.
2. Description of the Related Art
In advanced alternating-current elevator drives, the motor is generally controlled by means of a frequency converter, which is used to adjust the torque and rotational speed of the motor. An individual elevator travel may be regarded as consisting of a departure, acceleration, a constant-speed portion, deceleration and stopping at a landing. The motor is normally controlled by using a speed reference such that the elevator will follow a predetermined speed curve as accurately as possible. An important task in elevator operation is to stop the elevator car exactly at the landing without sudden speed changes or without a need to move the car in the reverse direction.
Usually when an elevator is to be stopped, constant deceleration is used, and just before the stop the deceleration is changed at a preselected rate of change or jerk to achieve a final rounding of the speed curve. This method works well if the elevator follows the speed reference accurately.
In prior art, there are solutions designed to make the elevator follow the speed curve as accurately as possible down to the final deceleration. Such a solution is described e.g. in international patent application PCT/FI97/00265. However, the solution disclosed in this publication is complicated and it can therefore not be applied in all elevator drives.
However, when torque control is used in an elevator, following the speed reference is difficult because the torque control determines the overall torque of the system. Increasing the gain increases the torque, but this leads to problems of stability.